Xerographic member



Nov. 29, 1960 .\\\\\\\\\\\\\\\\\\\\\\\\\\\\Y III I IIIIIIIIIIIIIIIIIII/II I R. M. SCHAFFERT 2,962,376

XEROGRAPHIC MEMBER I Filed May 14, 1958 INVENTOR. Roland M. Schaffert Ar TO/gA/Ey XEROGRAPHIC MEIVIBER Roland M. Schafiert, Vestal, N.Y.,assignor, by mesne assignments, to Haloid Xerox Inc., Rochester, N.Y., acorporation of New York Filed May 14, 1958, Ser. No. 735,302

8 Claims. (Cl. 96-1) This invention relates in general to xerography andin particular to a light sensitive plate therefor.

In the xerographic process which was first presented in US. 2,297,691 toC. F. Carlson, a photoconductive insulating surface on a relatively lowelectrical resistance backing is electrostatically charged in the dark,and then exposed to a light image. The charges leak oif rapidly to thebacking in proportion to which any given area is exposed. After suchexposure the coating is contacted with electrostatically charged markingparticles in the dark. These particles adhere to the areas where theelectrostatic charges remain forming a powder image corresponding to theelectrostatic image. The powder image can then be transferred to a sheetof transfer material resulting in a positive or negative print as thecase may be having excellent detail and quality. Alternatively, wherethe base plate is relatively inexpensive, as of paper or plastic, it maybe desirable to fix the powder image directly to the plate itself. Thepresent invention has as its scope and purpose the provision of a newsensitive member particularly useful in and adapted to xerography.

The photoconductive insulating coatings originally disclosed by Carlsoncomprised anthracene, sulphur, and various mixtures of these materialsas sulphur with selenium, etc. These materials have relatively low lightsensitivity. Consequently, there was an urgent need for improvedphotoconductive insulating materials. The dis covery of thephotoconductive insulating properties of highly purified vitreousselenium has resulted in this material becoming the standard incommercial xerography. The photographic speed of this material is manytimes that of the prior art photoconductive insulating materials.However, its spectral response is very largely limited to the blue ornear ultraviolet. Most light sources such as incandescent, photoflood,and sunlight have an appreciable if not major portion of their radiationin the red and far red portions of the spectrum. Thus, although vitreousselenium is enormously faster than prior art photoconductive insulatingmaterials, it uses only a small portion of the available radiant energy.Further, in many copying applications, as the reproduction of documentshaving printing, writing, or other information thereon in blue, or inmaking a color reproduction of a natural scene, the inability ofvitreous selenium to see larger wavelengths places a severe limitationon the utility of the vitreous selenium in the xerographic process. Now,in accordance with this invention there has been found a xerographiction having a shorter wavelength is absorbed in the very top-most layerof the selenium exposed to the incident radiation. Radiation having alonger wavelength, particularly red light, penetrates for a considerabledistance into the selenium before it is absorbed. Radiation of thiswavelength, that is, red light, does not have sufficient energy to raisean electron to the conduction band in vitreous selenium. Accordingly,its absorption adds nothing to the sensitivity of the selenium plate.

In the plate of the instant invention a layer of a red sensitivephotoconductor is positioned in the selenium layer within the range ofpenetration therein of red light. On exposure to activating radiationthe light having a wavelength shorter than about 600 millimicrons isabsorbed on or close to the top-most layer of vitreous seleniumoverlying the layer of red sensitive photoconductor generatinghole-electron pairs therein. The red light, penetrating to a greaterdepth, generates hole-electron pairs in the layer of red sensitivephotoconductor thus also contributing to the overall light response.

The plate of the instant invention has the red sensitive photoconductorpositioned in the selenium layeri.e., there is a layer of vitreousselenium both under and above the red sensitive photoconductor. This isillustrated in the drawing. The figure is a diagrammatic representationin section of an improved xerographic member ac cording to thisinvention. In the figure there is shown a new xerographic member 10comprising an electrically conducting backing 11 having a layer ofvitreous selenium 12 thereon, which is divided into a top layer 12A anda bottom layer 12B by an interlayer therein of a red sensitivephotoconductor 13. By presenting a layer of vitreous selenium to receivethe sensitizing charges placed on the free surface of thephotoconductive layer in the normal xerographic process, full use ismade of the excellent insulating properties in the dark of vitreousselenium thereby preventing smudging or obliviation as by lateralconductivity while positioning a layer of vitreous selenium between thered sensitive photoconductor and the conductive backing materialutilizes the excellent insulating properties of vitreous selenium toprevent injection of charge carriers from the backing material as willbe explained more fully hereafter. Thus the xerographic member accordingto this invention employs a red sensitive photoconductor (whichmaterials generally are substantially less electrically insulating thanvitreous selenium) without any substantial increase in dark decaytherefrom.

These novel xerographic members are further characterized by anincreased sensitivity to light and particularly as evidenced by a changein sensitivity specifically char acterized by a substantially increasedphotoconductive response to red light, i.e., radiation having awavelength greater than about 600 millimicrons. Further objects andfeatures of the invention. will become evident in the followingdescription of the invention.

The backing member is selected according toconventional requirements forthe xerographic art andgenerally comprises a metallic plate, cylinder,sheet, web-or the like, or other backing material havingstructuralcharacteristics and being electricallyconductive either by-its inherentnature or by having an electrically conductive material dispersedthroughout its volume or coated thereover. Suitably this backing membermay be a metallic member such as a member of aluminum, brass, magnesium,zinc, iron, steel, chromium or the like, or maybe of other electricallyconductive material such as electrically conductive glass (as glasscoated with tin oxide, indium oxide, or copper iodide), metallized paperor plastic, or the like, or electrically conductive resins, plastics orlike materials. According to a presently preferred embodiment of-theinvention, the backing member consists'of an aluminum plate or cylinderhaving a thin coating thereover of an aluminum oxide layer. Where it isdesired to expose the photoconductive insulating layer 12 through thebacking material 11, thereby necessitating the use of a transparentconductive backing, a particularly preferred backing comprises glasshaving a coating there-* on of tin oxide rendering the coated surfaceelectrically conductive.

Coated on at least one surface of the base member is the photoconductiveinsulating layer comprising a first layer of vitreous selenium 12B, thena thin layer of the red sensitive photoconductor 13, and then disposedon the surface of the red sensitive photoconductor another layer ofvitreous selenium 128. The layer of red sensitive photoconductor 13 isso positioned relative to the surface receiving the incident radiationas to be no more than about 20 microns from the surface thereof. Thus,if exposure is to be from the free surface of the vitreous selenium, thelayer of red sensitive photoconductor desirably is no more than about 20microns beneath the free surface. Conversely, if exposure is to bethrough the backing member 11 then the layer of red sensitivephotoconductor is no more than about 20 microns from the interfacebetween the vitreous selenium and the conductive backing member. It hasbeen found that the light absorption of amorphous selenium is such thatred light does not penetrate in any substantial amount to more thanabout 20 microns. Accordingly, in order for a significant amount ofactivating radiation to penetrate to the layer of red sensitivephotoconductor, layer 13 should not be separated by more than about 20microns of vitreous selenium from the activating radiation.

The conductive backing, by virtue of its being an electrical conductor,constitutes an almost infinite supply of charge carriers relative to thevitreous selenium. Accordingly, when exposure is to occur through theback, two considerations regulate the thickness of layer 12B: First,depending on the relative electron energy levels of the conductivebacking, the selenium and the red sensitive photoconductor, the redsensitive photoconductor may cause undesirable injection of chargecarriers into the bulk of the selenium of not sufficiently separatedfrom the conductive backing. Thus, if the red sensitive photoconductoris an n-type material (that is, has a larger number of negative chargecarriers than positive charge carriers) and positive charges are used tosensitize the selenium, the red sensitive photoconductor will causeinjection of electrons into the selenium thereby decreasing the abilityof the vitreous selenium to retain an electrostatic charge in the darkunless layer 1213 effectively separates layer 13 from backing 11.Secondly, as stated above, layer 12B should be no thicker than about 20microns when, as here, exposure is to be through the backing. Forexposure through the back, then, layer 12B should be from about 0.1 to20 microns thick and preferably is from about 1 to microns thick whilelayer 12A is thick enough to afford sufficient charge storage forutility in the Xerographic process, i.e., so that the thickness of theoverall photoconductive insulating layer is at least about 20 microns.

It has been found that while red light will penetrate vitreous seleniumto an appreciable distance, blue light (to which vitreous seleniumresponds) is highly absorbed by the red sensitive photoconductor.Consequently, if there is no layer of vitreous selenium interveningbetween the red sensitive photoconductor and the backing material whilethe red light falling on the red sensitive photoconductor is effectiveto create hole-electron pairs .in response thereto, little or not use ismade of the shorter wavelength radiations which are largely absorbed bythe red sensitive photoconductor without the corresponding creation ofhole-electron pairs thereby. Thus, the use of a red sensitive backinglayer coupled with exposure through the back merely acts to substitute ared sensitive plate for the previously blue sensitive plate whereas theplate of the instant invention creates a xerographic plate responding toboth the red and the blue portions of the spectrum.

Where exposure of the xerographic plate occurs through the free surfaceof the vitreous selenium, the red sensitive photoconductor may be asclose to the free surface of the vitreous selenium as desired so long asit does not interfere with the charge-holding capacities of theselenium. In general, it has been found desirable that the red sensitivephotoconductor should be covered with at least about one micron ofvitreous selenium. It is preferred that the thickness of layer 12B befrom about 2 to about 10 microns.

The red sensitive photoconductive materials which appear operable toconstitute layer 13 in the xerographic member of the instant inventionconsist of the sulfides, selenides and/o1 tellurides of antimony,arsenic, bismuth, cadmium, gallium, indium, lead, mercury and tellurium.While thallium has the requisite red sensitivity, it has been found thatelemental thallium causes undesirable crystallization in the selenium.Layer 13 may be formed by interrupting the vacuum evaporation of theselenium after the deposition of layer 12B and depositing the desiredred sensitive photoconductor, then continuing the evaporation of theselenium. Alternatively, the layer may be formed by evaporating thedesired compound during the evaporation of the selenium, timing theevaporation of the red sensitive photoconductor relative to the selenium(as by means of a shutter over the boat or boats containing the desiredcompound or compounds) to assure the deposition of layers 12B and 12A ofvitreous selenium. If desired the metal may be evaporated as such toform the selenide with the coevaporated selenium; or a mixedseleno-sulfide and/or telluride may be formed by coevaporation of sulfurand/ or tellurium with the metal and selenium. In this casestoichiometric equivalence is not essential, particularly in the case ofthe arsenic and tellurium. The layer of red sensitive photoconductor isusually in the order of 0.5 to 10 microns thick.

The total thickness of the layer of photoconductive insulator should befrom about 20 to 200 microns. Generally it is preferred that thephotoconductive layer be from about 20 to about microns thick. Thus, thexerographic plate according to the instant invention comprises aconductive backing material having on at least one surface thereof alayer of vitreous selenium from about 20 to 200 microns thick, saidselenium having therein a layer of red sensitive photoconductor at leastabout 1 micron from the surface of the selenium layer receiving theactivating radiation and no more than about 20 microns from thatsurface.

The preferred method of preparation of xerographic plates according tothe instant invention is by vacuum evaporation. In this process thebacking member, as an aluminum plate, is placed in a high vacuum (in theorder of about 1 micron of mercury pressure) and a layer of vitreousselenium evaporated thereon while maintaining the backing plate at atemperature of between about 60 and C. and preferably in the order ofabout 75 C. The deposition of the selenium layer is halted, as byplacing a shutter over the evaporation boat, when the layer reaches thedesired thickness. The layer of red sensitive photoconductor is thenevaporated onto the surface from a second evaporation source. When theevaporation of the red sensitive photoconductor is completed, theevaporation of the selenium is resumed and continued until the desiredthickness of selenium has been deposited. The layer of red sensitivephotoconductoris thinner than the vitreous selenium carrier layer andmay be as thin as 0.1 micron or as large as 50 percent of the thicknessof the selenium. It is usually not less than about 1 micron and no more.than about 20 percent of the thickness of the selenium layer. Desirably,it will be between about 0.5 and 10 microns thick. Other methods ofpreparation of the new Xerographic member according to this inventionmay be employed than the presently preferred method and the followingexamples are presented in illustration of but not in limitation of theinvention.

' Example I A brass plate having a smooth, flat surface is totallycleaned by scrubbing in water containing a detergent. The plate isrinsed clean and is then polished with a cleaning material which isunderstood and believed to have a hydrocarbon wax base and beingavailable under the trade name Glass Wax. This polish is applieduniformly over the surface of the plate and is vigorously polishedoifusing a clean dry cloth to leave a polished surface which isunderstood and believed to have an extremely'thin layer of a hydrocarbonwax material. The thus prepared plate is placed in an evacuation chamberin contact with a platen through which is circulated water at acontrolled temperature. The chamber is evacuated and the watertemperature set to control the temperature of the brass plate at 80 C.Selenium placed'in a molybdenum evaporation boat is then brought to aposition about 6" from the surface of the plate, and the seleniumevaporated onto the surface by heating from electrical heaters-locatedat the molybdenum evaporation boats. A shutter is then drawn over theboat to stop the evaporation when the selenium is about 15 microns thickand another molybdenum evaporation boat moved into evaporation position.A layer of mercuric sulfide about 2 microns thick is evaporated. Thisboat is then shuttered, and the evaporation of the selenium continued tocompletion placing an additional layer of vitreous selenium about 7microns thick on the plate. After the evaporation is completed the plateis removed from the evaporation chamber. The plate so prepared issuitable for xerography and characterized by having extended spectralresponse.

Example 2 An aluminum plate having a thin coating thereover of analuminum oxide layer is placed in the evaporation chamber used inExample 1 in contact with the platen thereof. No water is circulated inthe platen. The selenium is evaporated from an Alundum (a trademark ofthe Norton Co., for an alumina refractory) boat. After evaporationproceeds for about 25 minutes, the shutter is removed from a secondAlundum crucible and tellurium evaporated while the evaporation of theselenium proceeds. The boat containing the tellurium is heated beforethe shutter is removed, the shutter preventing the premature escape ofthe tellurium vapors. The rate of evaporation of the tellurium relativeto selenium is such that the layer formed during the coevacuation isestimated to contain about 45 percent tellurium based on the totalweight of the selenium and tellurium being evaporated at that time.After two minutes the shutter is replaced on the tellurium-containingboat, and the evaporation of the selenium completed five minutesthereafter. The resulting Xerographic plate has a photoconductive layerhaving a total thickness of about 50 microns. It is estimated, based onthe evaporation rate, that the thickness of the selenium-tellurium layeris about 3.5 microns while the selenium coating thereover is about 5microns thick. After the evaporation is completed the plate is removedfrom the vacuum chamber. The plate so prepared is suitable forxerography and is particularly characterized by having greatly extendedspectral response as compared to Xerographic plates containing onlyvitreous selenium.

Example 3 The procedure of Example 2 was repeated employing indiumtrisulfide in the second boat which is evaporated while the evaporationof the selenium continues. The total thickness of the photo-conductiveinsulating layer is again about 50 microns, the thickness of the layercontainingthe' indium'trisulfide is about 8 microns and the thickness ofthe overlying selenium layer about 2 microns. The percentage of indiumtrisulfide in the red sensitive photoconductive layer is about 18%. Thethus produced plate is characterized by increased Xerographicsensitivity to red light.

t f Example 4 The procedure of Example 2 is repeated using gallium inplace of tellurium. Layer 12B is about 20 microns, layer 12A about 3microns and layer 13 about 1 micron. The evaporation of gallium iscontrolled to give a concentration of -85% gallium triselenide by weightin layer 13. A plate of comparable quality and utility is produced.

Example 5 The procedure of Example 1 is repeated using antimonytrisulfide for the red sensitive photoconductor. A plate of comparablequality is produced.

The use of the red sensitive photoconductive layer in the midst of thevitreous selenium causes the creation of both holes and electrons uponthe absorption of activating' radiation, i.e., red light. Under theelectrostatic 'field applied across the layer of vitreous selenium byreason of the sensitizing charges thereon, both the holes and electronsmigrate through the vitreous selenium. It has been found that seleniumlayers conduct both electrons and holes but that the mobility for holesis approximately ten times that for electrons. Thus, before vitreousselenium can be used in the novel plates of the instant invention itshould be treated to increase the range therein of the minority chargecarriers.

It is presently believed that in vitreous selenium, the selenium formslong chains with an unpaired electron at each end. This unpairedelectron creates a possible electron trap. If two or more seleniumchains terminate in the same area, the result is likely to be a fairlydeep electron trap. Under favorable circumstances, oxygen and seleniumare isomorphous. If one of the chains terminates in oxygen, the trap islikely to be deeper due to the more electronegative character of oxygen.Accordingly, it has been found that if selenium is treated with anoxygen acceptor, and particularly one possessing two free (i.e.,valence) electrons, the material acts to remove oxygen and may alsosupply the need of the unpaired electron in the selenium chain thusfilling the trap. A satisfactory process for accomplishing this has beenfound to be adding iron filings to a crucible containing pellets ofXerographic grade selenium and evaporating the selenium under vacuum ata temperature well below the boiling point of iron. Such a treatmentfills the trap sites in the vitreous selenium to such an extent as toimpart a long range to electrons.

Xerographic members such as the ones above described may be used in theXerographic process with either positive or negative polarity charging,generally to a potential in the order of about to 800 volts, followed byexposure to an optical image whereby there is selective dissipation ofthe electrostatic charge. The resulting electrostatic image can bedeveloped, i.e. made visible, by treatment with an electroscopicmaterial and optionally the developed image is transferred to a supportmember to yield a Xerographic print.

It is to be understood that other methods of preparation of theXerographic member may be employed including melting and pressing orspraying molten selenium and/or red sensitive photoconductor onto theappropriate carrier layer.

I claim:

1. A Xerographic plate comprising an electrically conductive backingmember having on at least one surface a layer of photoconductiveinsulating material between about 20 and 200 microns thick, saidmaterial consisting essentially of vitreous selenium and having thereina layer between about 0.5 and 10 microns thick of a red sensitivephotoconductor, said photoconductor being positioned withinthe seleniumat least about 0.1 micron from said backing material and at least about'1 micron from the free surface of said selenium layer and no more thanabout 20 microns from the surface of the selenium receiving theactivating radiation.

2. A Xerographic plate comprising an electrically conductive backingmember having on at least .one surface a layer of photoconductiveinsulating material between about 20 and 200 microns thick, saidmaterial consisting essentially of vitreous selenium and having thereina layer between about 0.5 and 10 microns thick of a red sensitivephotoconductor selected from the group consisting of the sulfides,selenides and tellurides of antimony, arsenic, bismuth, cadmium,gallium, indium, lead, mercury, tellurium and mixtures thereof with eachother and with vitreous selenium, said photoconductor being positionedwithin the selenium at least about 0.1 micron from said backing materialand at least about 1 micron from the free surface of said selenium layerand no more than about 20 microns from the surface of the seleniumreceiving the activating radiation.

3. A xerographic plate comprising an electrically conductive backingmember having on at least one surface a layer of photoconductiveinsulating material between about 20 and 200 microns thick, saidmaterial consisting essentially of vitreous selenium and having thereina layer between about 0.5 and 10 microns thick of a red sensitivephotoconductor selected from the group consisting .of the sulfides,selenides and tellurides of antimony,

arsenic, bismuth, cadmium, gallium, indium, lead, mercury, tellurium andmixtures thereof with each other and with vitreous selenium, saidphotoconductor being at least about 2 microns within said vitreousselenium and no more than about 10 microns from the surface of theselenium receiving the activating radiation.

4. A xerographic plate according to claim 3 wherein said red sensitivephotoconductor is tellurium selenide.

5. A xerographic plate according to claim 3 wherein said red sensitivephotoconductor is arsenic selenide.

6. A Xerographic plate according to claim 3 wherein said red sensitivephotoconductor is gallium triselenide.

7. A xerographic plate according to claim 3 wherein said red sensitivephotoconductor is antimony trisulfide.

8. A xerographic plate according to claim 3 wherein said red sensitivephotoconductor is mercuric sulfide.

References Cited in the file of this patent UNITED STATES PATENTS2,687,484 Weimer Aug. 24, 1954 2,803,541 Paris Aug. 20, 1957 2,803,542Ullrich Aug. 20, 1957 Patent No. 2,962,376 November 29 1960 Roland M.-Schaffert error appears in the above numbered pat- It is herebycertified that etters Patent. should reades ent requiring correction andthat the said L *corrected'belowr second occurrence read Column 3, line42, for "of",

column 4L line if line 69, for not, read no 12, for "12B" read 12ASigned and sealed this 26th day of September 1961 SEA L) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of PatentsUSCOMM-DC-

1. A XEROGRAPHIC PLATE COMPRISING AN ELECTRICALLY CONDUCTIVE BACKINGMEMBER HAVING ON AT LEAST ONE SURFACE A LAYER OF PHOTOCONDUCTIVEINSULATING MATERIAL BETWEEN ABOUT 20 AND 200 MICRONS THICK, SAIDMATERIAL CONSISTING ESSENTIALLY OF VITREOUS SELENIUM AND HAVING THEREINA LAYER BETWEEN ABOUT 0.5 AND 10 MICRONS THICK OF A RED SENSITIVEPHOTOCONDUCTOR, SAID PHOTOCONDUCTOR BEING POSITIONED WITHIN THE SELENIUMAT LEAST ABOUT 0.1 MICRON FROM SAID BACKING MATERIAL AND AT LEAST ABOUT1 MICRON FROM THE FREE SURFACE OF SAID SELENIUM LAYER AND NO MORE THANABOUT 20 MICRONS FROM THE SURFACE OF THE SELENIUM RECEIVING THEACTIVATING RADIATION.